26 World War I spies for the United Kingdom

Aaron Aaronsohn was a Jewish agronomist, botanist, and Zionist activist, who was born in Romania and lived most of his life in the Land of Israel, then part of the Ottoman Empire. Aaronsohn was the discoverer of emmer, believed to be "the mother of wheat."

Sarah Aaronsohn was a member of Nili, a ring of Jewish spies working for the British in World War I, and a sister of notable agronomist Aaron Aaronsohn. She is often referred to as the "heroine of Nili."

Captain Stephen Alley was a British mechanical engineer and Secret Intelligence Service (MI6) agent in pre-revolutionary Russia who may have had an involvement in the murder of Rasputin in 1916 and in a plan to try to rescue the Russian Imperial Family, the Romanovs, imprisoned in Ipatiev House in 1918 by the Bolsheviks.

Gertrude Margaret Lowthian Bell, CBE was an English writer, traveller, political officer, administrator, and archaeologist who explored, mapped, and became highly influential to British imperial policy-making due to her knowledge and contacts, built up through extensive travels in Syria-Palestine, Mesopotamia, Asia Minor, and Arabia. Along with T. E. Lawrence, Bell helped support the Hashemite dynasties in what is today Jordan as well as in Iraq.

Louise Marie Jeanne Henriette de Bettignies was a French secret agent who spied on the Germans for the British during World War I using the pseudonym of Alice Dubois. She was posthumously awarded the Cross of the Legion of Honor, the Croix de guerre 1914-1918 with palm, and the British Military Medal, and she was made an Officer of the Order of the British Empire.

Lieutenant-Colonel Sir Claude Edward Marjoribanks Dansey, KCMG, also known as Colonel Z, Haywood, Uncle Claude, and codenamed Z, was the assistant chief of the Secret Intelligence Service known as ACSS, of the British intelligence agency commonly known as MI6, and a member of the London Controlling Section. He began his career in intelligence in 1900, and remained active until his death.

Anne Dawson MBE was a British secret agent in occupied Belgium in WW1. She worked for the GHQ Wallinger London unit in 1917–18, reporting to (Sir) Ivone Kirkpatrick who was based in Rotterdam. Anne Dawson was one of only two known British female agents operating behind German lines in Flanders, the other being Edith Cavell who had been a long time resident of Belgium prior to WW1.

:About the historian, born 1934, see Paul Dukes (historian).

Avshalom Feinberg was one of the leaders of Nili, a Jewish spy network in Ottoman Palestine helping the British fight the Ottoman Empire during World War I.

Armgaard Karl Graves acted as a double agent for the German naval intelligence service and the British MI5 before the start of the First World War. He was fired from the German service and called a "double-dyed rascal."

Lieutenant-Colonel Gerard Evelyn Leachman, CIE, DSO was an English soldier and intelligence officer who travelled extensively in Arabia.

René Charles Joseph Marie Lefebvre was a French factory-owner from Tourcoing, who died in the German concentration camp in Sonnenburg, in the Province of Brandenburg, where he had been imprisoned by the German Gestapo because of his work for the French Resistance and British Intelligence. René Lefebvre was the father of French Roman Catholic archbishop Marcel Lefebvre, the founder of the international Traditionalist Catholic organisation Priestly Fraternity of Saint Pius X (SSPX).

Yosef Lishansky was a Jewish paramilitary and a spy for the British in Ottoman Palestine. Upon his arrival in Palestine, Lishansky sought to join HaShomer but, denied membership, he founded a rival organization, HaMagen. Several years later, he joined the Jewish espionage organization, Nili. Lishansky was wanted by the Ottomans for his activities and was betrayed by HaShomer when he sought refuge with them. He escaped capture several times, but was eventually caught and sentenced to death in Damascus.

Sir Robert Hamilton Bruce Lockhart, KCMG was a British diplomat, journalist, author, secret agent, and footballer. His 1932 book Memoirs of a British Agent became an international bestseller and brought him to the world's attention by telling of his failed effort to sabotage the Bolshevik Revolution in Moscow in 1918. His co-conspirators were double agents working for the Bolsheviks. In the end, the "Lockhart Plot" was revealed as a cunning sting operation controlled by Felix Dzerzhinsky with the goal of discrediting the British and French governments.

William Somerset Maugham was an English playwright, novelist, and short-story writer. He was among the most popular writers of his era and reputedly the highest-paid author during the 1930s.

William Melville was a British law enforcement officer and the first chief of the British Secret Service Bureau.

NILI was a Jewish espionage network which assisted the United Kingdom in its fight against the Ottoman Empire in Palestine between 1915 and 1917, during World War I. NILI is an acronym which stands for the Hebrew phrase "Netzah Yisrael Lo Yeshaker", which translates as "the Eternal One of Israel will not lie". The British government code-named NILI the "A Organization", according to a 1920 misfiled memorandum in the British National Archives, as described in the book Spies in Palestine by James Srodes.

Conrad Fulke Thomond O'Brien-ffrench, 2nd Marquis de Castelthomond was a distinguished British Secret Intelligence Officer, Captain in the Tipperary Rangers of the Royal Irish Regiment and 16th The Queen's Lancers in World War I, and Mountie for the Royal North-West Mounted Police. He was also an accomplished artist, linguist, mountaineer, skier, and author.

Gabrielle Alina Eugenia Maria Petit was a Belgian woman who spied for the British Secret Service during World War I. She was executed in 1916, and became a Belgian national heroine after the war's end.

Harry St John Bridger Philby, CIE, also known as Jack Philby or Sheikh Abdullah, was a British Arabist, adviser, explorer, writer, and Colonial Office intelligence officer.

Oswald Rayner was a British Secret Intelligence Service (MI6) agent in Russia during the First World War. He is believed by some to have been involved in the final murder plot against Grigori Rasputin, but "the archives of the British intelligence service (MI6) do not hold a single document linking Rayner, Hoare, or any other British agent or diplomat to the murder."

Sidney George Reilly — known as "Ace of Spies" — was a Russian-born adventurer and secret agent employed by Scotland Yard's Special Branch and later by the Foreign Section of the British Secret Service Bureau, the precursor to the modern British Secret Intelligence Service (MI6/SIS). He is alleged to have spied for at least four different great powers, and documentary evidence indicates that he was involved in espionage activities in 1890s London among Russian émigré circles, in Manchuria on the eve of the Russo-Japanese War (1904–05), and in an abortive 1918 coup d'état against Vladimir Lenin's Bolshevik government in Moscow.

Admiral Sir Hugh Francis Paget Sinclair,, known as Quex Sinclair, was a British intelligence officer. Between 1919 and 1921, After World War I, he was Director of British Naval Intelligence, and he helped to set up the Secret Intelligence Service before World War II.

Sir Mansfield George Smith-Cumming was a British naval officer and the first director of the Secret Intelligence Service (SIS).

Bertrand Stewart worked as a solicitor in London and was also a military officer in the Queen's Own West Kent Yeomanry, he fought in the Second Boer War and the First World War. In between the two wars he volunteered to spy on German naval actions. He was famously arrested in Germany on 2 August 1911 and sentenced to four years in prison. Stewart and another British spy, Captain Trench, were pardoned and released by the German Kaiser as a present to Ernest Augustus the Duke of Brunswick when Augustus married the Kaiser's daughter, Princess Victoria Louise of Prussia. He died fighting off a German attack near the River Vesle during the Battle of the Marne.

Sir Charles Leonard Woolley was a British archaeologist best known for his excavations at Ur in Mesopotamia. He is recognized as one of the first "modern" archaeologists who excavated in a methodical way, keeping careful records, and using them to reconstruct ancient life and history. Woolley was knighted in 1935 for his contributions to the discipline of archaeology. He married the British archaeologist Katharine Woolley.